Advanced technological uses for wire and cable have imposed many new requirements upon traditional wire and cable specifications and functions. In missile and aerospace environments, for example, the need for light-weight cabling is directly related to aircraft performance, as well as operating cost. Wiring is also often required to meet stringent shielding specifications, since a missile or aircraft must fly through radiation and electrical interference fields without compromising the on-board electronics.
Wire and cables are currently shielded electrically by braiding shields of wire mesh about the primary wire core and insulation. This shielding is meant to prevent RFI and EMI disturbances from influencing the signals in the cable.
As advanced technology requirements impose greater stringency on shielding and weight specifications, the previously functional braided articles become less acceptable. Shielding leakages occur in these conventional cables, owing to the looseness with which the wire mesh is braided, creating holes in the shield web. In addition, the stiffness of the braided metal wire makes it difficult to conform the mesh to the surfaces of the insulation and core and thus leaves small gaps. Such gaps limit the frequency range in which the cable or wire can be operationally effective. While it may be possible to use a finer wire mesh to resolve some of the above-mentioned shielding problems, it is still necessary to contend with the low-weight requirements that these environments impose. The low-weight requirements cannot be practically met by utilizing the conventional wire mesh braiding techniques.
U.S. Pat. No. 5,103,067 issued on Apr. 7, 1992, to Mahmoud Aldissi and having a common assignee, for SHIELDED WIRE AND CABLE taught that the shielding of wire and cable could be greatly enhanced by the use of metal-coated high-strength fibers woven into a shield layer.
The present invention expands upon the teachings of the aforementioned patent, with new shielded wire and cable articles using high-performance liquid crystalline polymers such as poly (p-phenylene-2,6-benzobisthiazole) [hereinafter referred to as PBT], polybenzoxazole (PBO), polybenzimidazole (PBI), polyester-polyarylate and polyester-polyarylate commingled with glass fibers. The new wire and cable articles of this invention further suggest the use of shielding fibers that comprise ceramic materials, such as silicon carbide and carbon-coated silicon carbide. The invention also contemplates fibers consisting of bridged macrocyclic metal complexes and hybrids, such as poly-phthalocyanines. The fibers may also include inherently conductive materials such as polythiophenes and polyanilines. All of the fibers are coated with a thin metallic layer of silver, copper or nickel. Thereafter, the metal-coated fibers may be woven, braided or served into a mesh or shield layer to provide shielding in frequency ranges of approximately between 100 KHz and 1 GHz or greater.
The high-tensile strength and the flexibility of the fibers of this invention guarantee that the fibers can be made thin without losing their structural integrity. The thinner the fiber, the more tightly it can be braided or woven, hence, the greater its shielding effectiveness. The greater flexibility of the fiber mesh, as compared to the wire mesh, also creates a more comprehensive conformity of the mesh to the surface of the underlying insulation. Such improved conformity further improves the closeness and tightness of the mesh shield. This also improves shielding at a higher frequency range.
The fibers have an obvious weight advantage over that of metallic wire, thus providing a solution to a vexatious aspect of the new aerospace specifications.
In addition to the advantages of improved shielding and weight, many of the materials of this invention also enhance the operating temperature range. For example, the fiber materials that are admixed with ceramic and silicon carbide, and the mixture of polyester-polyarylate commingled with glass fibers will each provide a shield having a temperature that can exceed 150.degree. C.